This invention relates to an axial flow turbine such as, for example a steam turbine and, a gas turbine, more particularly, to a turbine stage structure constructed of a stationary blade row and a moving blade row.
A conventional turbine stage structure includes a row of stationary blades arranged annularly between a stationary outer wall and a stationary inner wall and a row of moving blades radially provided on a rotor disc. The moving blades have shroud ring fixed to the tips thereof. A labyrinth sealing is formed of a plurality of fins arranged in an annular space defined by the inner surface of the stationary outer wall and the shroud ring in order to minimize leakage of working fluid through the space.
In a large-sized turbine provided with several above-mentioned stages to get large output, a difference in thermal expansion occurs between the stationary wall and the rotor during a transitional period of the turbine operation such as starting and stopping. In order to prevent the stationary outer wall and the rotor from being damaged through contact, it is necessary, in a normal operation to maintain a large axial gap between an axial end of the shroud ring and an axial end portion of the stationary outer wall facing the shroud ring axial end. The difference in thermal expansion between the stationary outer wall and the rotor increases in proportion to a rise in steam temperature and pressure and an increase in machine size, so that turbines of large capacity have a large axial gap as compared with small capacity turbines.
Influence of the axial gap on stage effeciency is disclosed, for example, in Thermal Engineering Vo. 20 (1) of 1973 "The Influence of Blade Clearance on the Characteristics of a Turbine stage" by I. G. Gogolev, et al. and Thermal Engineering Vol. 20 (3) of 1973 "Comparative Tests of Pressure Stage by Two Simulaton Methods" by A. S. zil' Berman, et al. According to these publications, the turbine stage efficiency decreases as the axial gap increases. Generally, the influence of the axial gap on the efficiency can be expressed as function of ratio of the axial gap to blade length, namely, the turbine stage efficiency decreases as the blade length reduces or as the axial gap increases.
The cause of and the mechanism for reduction of efficiency due to the axial gap between the axial end of the shroud ring and the axial end of the stationary wall facing the shroud ring end has not been full understood, and it has been through that, in a high pressure turbine, the decrease in turbine efficiency occurs inherently, and consequently, no effective improvement of turbine efficiency has been proposed.
On the other hand, since the reduction of steam leakage from the tip of the moving blade, that is, the reduction of the steam leakage from the spacing between the shroud ring and the seal fins is effective for raising the turbine efficiency, there have been various measures such as increasing the number of the fins used, minimization of the radial clearance and use of the shroud ring of complicated, stepwise shape such as is disclosed in, for example, Japanese Patent Publication No. 45726/1980. According to "Non-contact sealing theory" by Kazuo Komodori, Corona Publishing Co., in the above-mentioned sealing portion, it is necessary for expansion chambers defined by the fins to have a volume to prevent leakage by causing effectively eddy loss in the expansion chamber. Therefore, it is necessary to make fins longer in length and practical steam turbines for power plants use the fins of about 10 mm length.
These steam turbines each have, at the upstream outer side of the shroud ring, an expansion space defined by an axial end face of the stationary wall facing the shroud ring, a stationary surface facing the outer surface of the shroud ring and provided with the sealing fins, and the fin at the most upstream side. Small-sized steam or gas turbines and low pressure stages of large-sized turbines can not mount the shroud rings without causing strength problems, and, consequently, such construction can not attain an effect of sealing fins. In such a case, the stationary wall is very close to the tops of the moving blades and the expansion space is small.
In general, however, high or medium pressure stages have a shroud ring to prevent decrease in efficiency, and sealing fins, with thin tips are employed so as to prevent serious damage when the shroud ring contacts the stationary wall. In such a construction, the expansion space becomes relatively large as the axial gap increases. In some cases, an increase of leakage from the blade tips due to the enlargement in the axial gap results in a decrease in stage efficiency. In such cases, the gap serves as a part of the sealing. When the axial gap is nearly equal to the radial gap in the vicinity of the sealing portion, the steam leakage at the tip of the moving blade increases as the the axial gap increases. The leakage, however, does not almost change according to the value of the axial gap when the axial gap is larger than twice the amount of the radial gap. According to the experimental results, it was determined that, even if steam leakage at the tip of the moving blade is very small with the radial gap being made very small, the turbine efficiency decreases greatly as the axial gap increases, and a large loss corresponding to several times as large as loss due to the steam leakage.
Therefore, the only prevention of the steam leakage at the tip of the moving blade is not a decisive measure for preventing the decrease in turbine stage efficiency due to an increase in the axial gap.
It was also experimentally determined that a principal cause of the decrease in turbine stage effeciency caused by an increase of the axial gap at the blade tip is an action of fluid in the axial gap or expansion space, however, presently there are no known publications in which the above-mentioned cause is disclosed.
In Japanese Patent Laid-Open No. 128008/1975, thin members are disposed generally axially in the axial gap at the blade tip with the fin members forming a plurality of passages for fluid therebetween, so as to guide the fluid so as to flow along the passages, whereby the rotor is prevented from flow-induced vibration.
This construction does not prevent the decrease in the turbine stage efficiency caused by the enlargement of the axial gap.
An object of the invention is to provide an axial flow turbine in which a decrease in turbine stage caused by enlargement of an axial gap between an axial end of a shroud ring and a stationary wall, facing the axial end of the shroud ring is prevented.
Experiments have indicated that a decrease in turbine stage efficiency, caused by enlargement of an axial gap between an axial end of a shroud ring and a stationary wall facing the axial end, occurs because of working fluid circulation which is such that a partial flow, branched from a main stream having passed through a stationary blade row, enters an expansion space formed immediately downstream of the axial gap with respect to a fluid passage formed between said stationary wall and the shroud ring to cause an eddy loss and a windage loss thereby consuming the kinetic energy. Most of the partial flow flows into and mixes with the main stream to thereby reduce kinetic energy of the main stream, and there is an increase in disturbance of the main stream caused by the circulation.
In accordance with the present invention, means are provided in the expansion chamber, for preventing the above-mentioned fluid circulation.
According to the present invention, the means for preventing the fluid circulation includes an annular solid substance provided in the expansion space immediately downstream of the axial gap between the axial end of the shroud ring and the stationary wall facing the axial end.